Recently, in a mobile communication system (e.g., Personal Handyphone System: hereinafter referred to as PHS) that has been developing rapidly, PDMA (Path Division Multiple Access) scheme is proposed, in which radio terminal apparatuses (terminals) of a plurality of users can be connected to a radio base station apparatus (base station) in a spatial multiplexing manner, by spatially dividing an identical time slot of an identical frequency for improving the frequency effectiveness of a radio wave.
In this PDMA scheme, an adaptive array technique is currently employed, in which an uplink signal from each user's terminal antenna is received by an array antenna of a base station and extracted with reception directivity through an adaptive array process. A downlink signal to the terminal from the base station is transmitted from the array antenna with transmission directivity to the antenna of the terminal.
Such an adaptive array process is well known in the art and described in detail, for example, in “Chapter 3: MMSE Adaptive Array” in “Adaptive Signal Processing by Array Antenna”, Nobuyoshi Kikuma, Kagaku Gijutsu Shuppan, pp. 35-49. Thus, description on its operation principle is not given herein. Further, specific arrangement of a radio apparatus employing the adaptive array processing is well known in the art, as disclosed in detail in International Publication No. WO00/79702 of the present applicant.
In the following, a base station that performs downlink transmission directivity control using such an adaptive array process is referred to as an adaptive array base station.
As for a terminal, one performing selective diversity reception (hereinafter referred to as diversity reception) using a plurality of antennas is known. In reception, such a terminal operates to select one antenna with higher reception level as a reception antenna from, for example, two antennas. Such a conventional diversity reception terminal performs the above mentioned diversity reception regardless of whether the base station to be connected is an adaptive array base station performing transmission directivity control or a nondirectional base station.
FIGS. 26A and 26B schematically show the connection states between terminals and adaptive array base stations, FIG. 26A showing the connection state between terminals not performing diversity reception and adaptive array base stations, and FIG. 26B showing the connection state between terminals performing diversity reception and adaptive array base stations.
Referring to FIG. 26A, as indicated by a bold arrow, a terminal 3 not performing diversity reception is connected to a desired adaptive array base station 1, and from an array antenna of adaptive array base station 1, a downlink signal is transmitted with transmission directivity to one antenna of terminal 3 that has transmitted an uplink signal. A hatched region D (D: Desired) indicates a state where a beam of signal wave is directed to terminal 3 from adaptive array base station 1.
In this case, terminal 3 can receive the downlink signal from adaptive array base station 1 at the maximum power due to the transmission directivity of desired adaptive array base station 1. The relationship between a terminal 4 not performing diversity reception and its desired adaptive array base station 2 is the same.
Though the signal wave from adaptive array base station 2 as indicated by a broken arrow U (U: Undesired) functions as an interference wave for terminal 3, as can be seen from the emission state of the signal wave in FIG. 26A, the signal power of the interference wave from adaptive array base station 2 received at terminal 3 is of the minimum. The relationship between terminal 4 not performing diversity reception and its non-desired adaptive array base station 1 is the same.
As above, an excellent connection state with less interference can be realized between a terminal not performing diversity reception and a desired adaptive array base station.
On the other hand, referring to FIG. 26B, for example a terminal 5 performing diversity reception transmits an uplink signal from one antenna 5a, establishing a connection relationship with adaptive array base station 1 as indicated by a bold arrow, similarly to the relationship shown in FIG. 26A. Hence, at antenna 5a of terminal 5, a downlink signal from desired adaptive array base station 1 is received at the maximum power, while a transmission signal from a non-desired adaptive array base station 2 (an interference wave) indicated by a broken (fine) arrow is received at the minimum power.
Since beam of signal wave D from desired adaptive array base station 1 is not directed to the other antenna 5b of terminal 5 not transmitting an uplink signal, the power of reception signal from adaptive array base station 1 decreases. Therefore, the power of interference wave U from non-desired adaptive array base station 2 indicated by a broken (bold) arrow relatively increases at antenna 5b. 
The similar state occurs at antennas 6a and 6b of a terminal 6 performing diversity reception in the relationship with adaptive array base stations 1 and 2.
The similar problem arises in a spatial multiplexing base station realizing spatial multiple connection using such an adaptive array process. FIGS. 27A and 27B schematically indicate the connection states between terminals and base stations; FIG. 27A showing connection state between terminals not performing diversity reception and a spatial multiplexing base station, FIG. 27B showing connection state between terminals performing diversity reception and a spatial multiplexing base station.
Referring to FIG. 27A, as indicated by a bold arrow, terminals 30 and 40 not performing diversity reception are connected in spatial multiplexing manner to a desired base station 10 through the adaptive array process, and from an array antenna of spatial multiplexing base station 10, a downlink signal is transmitted with transmission directivity to one antenna of each of terminals 30 and 40 that has transmitted an uplink signal. A hatched region D indicates a state where a beam of signal wave is directed to each of terminals 30 and 40 from spatial multiplexing base station 10.
In this case, terminals 30 and 40 each can receive the downlink signal from base station 10 at the maximum power due to the transmission directivity of the desired base station 10.
As above, an excellent connection state with less interference can be realized between a terminal not performing diversity reception and a desired spatial multiplexing base station.
On the other hand, referring to FIG. 27B, for example a terminal 50 performing diversity reception transmits an uplink signal from one antenna 50a, establishing a connection relationship with spatial multiplexing base station 10 as indicated by a bold arrow, similarly to the relationship shown in FIG. 27A. Hence, at antenna 50a of terminal 50, a downlink signal from desired spatial multiplexing base station 10 is received at the maximum power.
Since beam of signal wave D from desired spatial multiplexing base station 10 is not directed to the other antenna 50b of terminal 50 not transmitting an uplink signal, the power of reception signal U from spatial multiplexing base station 10 decreases. Therefore, the power of interference wave from a non-desired base station that is not shown relatively increases at antenna 50b. 
The similar state occurs for antennas 60a and 60b of a terminal 60 performing diversity reception.
As above, at a terminal performing diversity reception, regardless of whether a desired base station is an adaptive array base station controlling downlink transmission directivity, one antenna with higher reception level is selected from two antennas as a reception antenna. Therefore, for example in a terminal 5 of FIG. 26B, when a combined power of a low reception power from desired adaptive array base station 1 and a large interference wave U from non-desired adaptive array base station 2 received at antenna 5b not transmitting an uplink signal exceeds the reception power from a desired adaptive array base station 1 received at antenna 5a that has transmitted an uplink signal, antenna 5b is selected as a reception antenna.
In this case, the signal received at antenna 5b has a large power of interference wave U from non-desired adaptive array base station 2 relative to the downlink reception signal from desired adaptive array base station 1, and hence it is a signal with a large interference component, i.e., a signal with low so-called DU ratio (Desired user's power:Undesired user's power).
Even when an attempt is made to demodulate such a reception signal with low DU ratio at terminal 5, an error occurs in a frame of a demodulation signal and a correct demodulation fails. In special, when the power level of a downlink signal (interference wave) U from non-desired adaptive array base station 2 increases, in the worst case, terminal 5 may inappropriately demodulate a downlink signal that is transmitted to another user's terminal 6 from adaptive array base station 2.
Similar problem arises in the spatial multiplexing base stations shown in FIGS. 27A and 27B.
Thus, in the conventional mobile communication system, when a terminal that performs diversity reception is connected to an adaptive array base station (or a spatial multiplexing base station), the DU ratio at the terminal decreases, and its reception performance is degraded by an interference wave. Accordingly, there exist a problem that the effect of the adaptive array technique to reduce the frequency reuse distance (the minimum distance between base stations that can share the same frequency) is reduced.
It is noted that, as a terminal of a mobile communication system, in addition to the above-mentioned diversity terminal, there is a terminal of the type that performs transmission and reception with one antenna. Further, a terminal that is capable of performing adaptive array reception is possible. As for a base station, in addition to the base station that controls the downlink transmission directivity such as the above-mentioned adaptive array base station, there is also a nondirectional base station that does not perform such a directivity control.
In such a mobile communication system, it is necessary to cause the terminal to perform the reception operation that is adapted to a transmission operation of the base station as much as possible, and the examples in FIGS. 26A-27B only illustrate the cases where the reception operation of the terminal is not adapted to the transmission operation of the base station.
Therefore, an object of the present invention is to provide a radio base station apparatus, a radio terminal apparatus, a mobile communication system, and a reception operation control program thereof, which are capable of causing the terminal to perform a reception operation adapted to a transmission operation of the base station as much as possible.
Another object of the present invention is to provide a radio base station apparatus, a radio terminal apparatus, a mobile communication system, and a reception operation control program thereof, in which the reception performance of the terminal is not degraded even when the terminal is connected to the base station that controls downlink transmission directivity, such as an adaptive array base station or a spatial multiplexing base station.